In recent years, introduction of renewable energy represented by wind power generation, photovoltaic power generation, and the like is making rapid progress. However, when a large amount of power generated by the renewable energy flows as reverse power flow into a power system, the voltage and frequency of the power system fluctuate, and the quality maintenance of the power is difficult. Consequently, in power systems at present, the acceptable renewable energy is often limited to suppress deterioration in the quality of power. That is, the power systems at present have a problem such that a large amount of renewable energy cannot be introduced.
To solve the problem and introduce a large amount of renewable energy, a novel power network in which autonomously-controlled power systems are disposed so as to be dispersed is being examined. One of such power networks is a smart grid. In power systems disposed so as to be dispersed, that is, in a grid, by absorbing fluctuations of the amount of power generation by the renewable energy by using a power storage device in the grid or a demand-supply balance adjusting function, reverse power flow to an existing power system can be largely reduced.
Further, a technique of enabling power to be interchanged among grids is proposed. According to the technique disclosed in Patent Literature 1, a plurality of multi-terminal asynchronous interconnected devices having a plurality of self-excited power converters for bidirectionally converting power are disposed and mutually connected, thereby constructing a power network. The power network system is controlled so that the sum of inflow power and outflow power in a power router will be zero, and interchanges powers asynchronously among the grids. With the configuration, power can be supplied from another grid at the time of power shortage, and robustness of the grids improves. Since the power can be interchanged among the grids, it is possible to perform power trades.